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Prime Minister John Key says he is committed to staying for the full three-year term if he remains as prime minister after the general election this year.

There has long been speculation that Mr Key would stand down part way through any third term, but Mr Key told Newstalk ZB that if his caucus and party still supported him he did intend to stick it out despite earlier thinking three terms could be too long.

Asked if he would stay on to fight the next election in 2017, he said the National Party caucus would decide that. However, it was his intention to stay on for the full term.

"I'd have to be sure I had the confidence of my ministers and all that sort of stuff, but if you're asking me 'am I committed for three years?' Yes."

He said if he had been asked in 2008 privately whether he would have served through to 2017 "I would have said, 'gosh, that seems an awfully long time."

"But when you're there, I don't know if there's something seductive about the job or whatever it is, but when you're there it goes really quickly. It goes a lot more quickly than I thought."

Asked by Leighton Smith if it was the attraction of power, he said no. "It's the capacity to make change and see it rolling out. You get a combination of slight exhaustion, sometimes frustration because it's MMP and you can't do everything you want ... But I must admit I sit there at the start of the sixth year looking at all this change and say 'the economy is getting back into surplus, debt levels are much less than they thought it would be, the country is growing, HSBC is calling us the 'rock star economy,' crime is at a 30-year low.

"And in my heart I sit there and go 'I reckon we've done a pretty good job. There's something energising about that."

Mr Key also ruled out any chance that a coalition with NZ First could result in its leader Winston Peters becoming prime minister for a portion of the term if he did stand down.